The present invention relates preferably to doors made of wood and used as entrance doors and as apartment frontdoors. In such doors striking plates are used beside the bolts to prevent a locked door from being broken up. On the hinge side of the dowels are generally used which may be located in the door itself or in the opposite frame part. The purpose of these dowels is also to make breaking-in more difficult. Despite these deterrents it is still possible to destroy a locked door, enabling illicit entry. In order to make this more difficult, doors have been made entirely of sheet metal and such doors certainly prevent undesired entry through the doorway. However, a metal door is considerably more expensive than a wooden door and it is thus not always feasible to replace a wooden door by a metal one.